Protein Synthesis

The creation of proteins by cells that uses DNA, RNA and various enzymes.

Supplement

It includes processes such as:

• Amino acid synthesis, a set of biochemical processes that produce amino acids from carbon sources like glucose. (Not all amino acids are produced by the body, other amino acids are obtained from diet)
• Transcription, the process by which an mRNA template, encoding the sequence of the protein in the form of a trinucleotide code, is transcribed from DNA to provide a template for translation
• Translation, the process in which amino acids are linked together in a specific order according to the rules specified by the genetic code

Transcription is the synthesis of mRNA from a DNA template.

It is like DNA replication in that a DNA strand is used to synthesize a strand of mRNA.

Only one strand of DNA is copied.

A single gene may be transcribed thousands of times.

After transcription, the DNA strands rejoin.

RNA polymerase recognizes a specific base sequence in the DNA called a promoter and binds to it. The promoter identifies the start of a gene, which strand is to be copied, and the direction that it is to be copied.

RNA polymerase unwinds the DNA.

RNA polymerase assembles bases that are complimentary to the DNA strand being copied. RNA contains uracil instead of thymine.

A termination code in the DNA indicates where transcription will stop.

The mRNA produced is called a mRNA transcript.

In eukaryotic cells, the newly-formed mRNA transcript (also called heterogenous nuclear RNA or hnRNA) must be further modified before it can be used.

A cap is added to the 5’ end and a poly-A tail (150 to 200 Adenines) is added to the 3’end of the molecule.

Eukaryotic genes contain regions that are not translated into proteins. These regions of DNA are called introns and must be removed from mRNA. Their function is not well understood.

The remaining portions of DNA that are translated into protein are called exons. After intron-derived regions are removed from mRNA, the remaining fragments- derived from exons- are spliced together to form a mature mRNA transcript.

Translation is the process where ribosomes synthesize proteins using the mature mRNA transcript produced during transcription. 

The diagram below shows a ribosome attach to mRNA, and then move along the mRNA adding amino acids to the growing polypeptide chain.

A mature mRNA transcript, a ribosome, several tRNA molecules and amino acids are shown. There is a specific tRNA for each of the 20 different amino acids.

Below: A ribosome attaches to the mRNA transcript.

A tRNA molecule transports an amino acid to the ribosome. Notice that the 3-letter anticodon on the tRNA molecule matches the 3-letter code (called a codon) in the mRNA. The tRNA with the anticodon "UAC" bonds with methionine. It always transports methionine. Transfer RNA molecules with different anticodons transport other amino acids.

A second tRNA molecule bonds to the mRNA at the ribosome. Again, the codes must match.

A bond is formed between the two amino acids.

The tRNA bonded to methionine drops off and can be reused later.

The ribosome moves along the mRNA to expose another codon (GAU) for a tRNA molecule.

The only tRNA molecule that can bond to the GAU site is a molecule with a CUA anticodon. Transfer RNA molecules with CUA anticodons are specific for asparagine.

Asparagine is now added to the growing amino acid chain